Components of desalination systems often accumulate mineral deposits resulting from minerals present in the liquid being processed. For example, Ca++ ions combine with HCO3− ions to form CaCO3 particles. Mineral deposits form in liquids in a variety of ways. Some mineral ions combine in the liquid stream and form particles that settle onto surfaces in the form of a soft loose sludge. This is sometimes called particulate fouling. In other instances, ions deposit out of solution at a heat transfer surface and form hard crystalline deposits or scaling that binds to the heat transfer surface. This latter phenomenon is often referred to as crystallization or precipitation fouling.
Scaling can create significant problems, particularly in heat exchangers and other desalination equipment where the treated liquid contacts hot surfaces. The solubility of mineral compounds in water, such as CaCO3, decreases as the liquid increases in temperature. This is sometimes referred to as inverse solubility. As a result, when water enters a heat exchanger and increases in temperature, dissolved mineral ions in the water deposit out of solution at the heat transfer surface where the water is the hottest. The mineral ions often adhere directly to the heat transfer surface as they react with HCO3− ions. In the case of calcium ions, the reaction may be expressed as:Ca+++2HCO3→CaCO3+H2CO3→CaCO3+H2O+CO2↑.
Since the formation of CaCO3 occurs on the heat transfer surface, the CaCO3 particles frequently bind to the heat transfer surface to form scale. Excessive scaling can damage heat exchangers and reduce the rate of heat transfer through the heat transfer surface. In extreme cases, scaling can permanently damage the desalination equipment.
Mineral deposits in fluid conduits and equipment require periodic removal. Brush punching tools that have a coarse scrubbing surface are adequate to remove softer mineral deposits formed by particulate fouling. However, brush punching is not effective to remove scaling caused by crystallization fouling, and thus additional cleaning measures must be used. For example, chemical cleaning with acid solutions is often used in conjunction with brush punching to remove hardened scale from heat transfer surfaces. These techniques are time consuming and labor intensive, requiring the equipment to be shut down for significant periods of time.
In the present state of the art, physical water treatment (PWT) methods are used to reduce scaling in heat transfer equipment. These methods use a variety of mechanisms, including permanent magnets, solenoid-coils, pressure drop devices, and vortex flow devices. Although these methods employ different technologies, they are all used to promote bulk precipitation of mineral particles at locations other than at heat transfer surfaces. The precipitation reduces the dissolved concentration of mineral ions that enter the heat exchanger, reducing the potential for scale formation on the heat exchangers. In the case of calcium ions, PWT methods are typically designed to enhance the molecular attraction between Ca++ and HCO3− ions to cause precipitation of CaCO3 particles at a desired location before the calcium ions can make contact with the heat transfer surface.
In PWT methods, the aim is to encourage the formation of soft sludge on the heat transfer surface through particulate fouling, and prevent hardened deposits at the heat transfer surface formed by crystallization fouling. Mineral ions are precipitated out of solution at locations other than at heat transfer surfaces to form seed particles in the bulk liquid. This reduces the concentration of dissolved mineral ions entering the heat exchanger, and thereby decreases the potential for mineral scaling on the heat transfer surfaces. As seed particles made of mineral ion precipitations enter the heat exchanger, they attract additional dissolved mineral ions that precipitate out of solution as the water temperature increases inside heat transfer equipment. The seed particles combine with the dissolved mineral ions to form relatively large particles that can be easily removed from the liquid stream. Particles that settle out of the liquid at a sump tank of a cooling tower and removed periodically via a drain hole at the sump tank. Alternatively, the particles form a soft sludge on the surface of heat exchanger tubes through particulate fouling. This sludge may be easily removed by punch brushing during a periodic maintenance program, or by scouring in areas having a higher water velocity.
In many prior art PWT methods an electrical field is employed to enhance the molecular attraction between Ca++ ions and HCO3− ions toward one another. One or more electrodes are placed on the exterior of a pipe or container that may be used to generate an indirect electrical field in the water. Note that the electrodes do not contact the liquid in the pipe or container Indirect electric fields, however, have limited effectiveness in reducing mineral scale, because they generally do not provide a strong enough electric field in the water to efficiently induce bulk precipitation. For example, it is known to surround a liquid carrying conduit with a solenoid coil driven by an alternating polarity in a square-wave current signal to induce a pulsating (reversing) electric field within the liquid. The electric field in the water is governed by Faraday s Law. According to Faraday s law, the electric field E is described by:
      ∫          E      ·              ⅆ        s              =            -              ∂                  ∂          t                      ⁢          ∫              B        ·                  ⅆ          A                    where E is an induced electric field vector, s is a line vector in the electric field, B is a magnetic field strength vector, and A is the cross sectional area of the solenoid coil. In this arrangement, an induced electric field is produced within the water, but the field typically has limited electric field strength. When the solenoid is driven by a square-wave voltage signal having a voltage of 12 volts, 5 amperes peak, and a frequency of 500 Hz, the electric field strength is not more than about 5 mV/cm.
Under Faraday s law, the strength of the induced electric field depends on the solenoid coil diameter. The electric field strength induced in the water generally decreases as the diameter of the pipe increases. Therefore, to provide adequate field strength in larger pipes, larger solenoid coil diameters must be used, thereby increasing material and energy costs.
The strength of the induced electric field is also dependent on the frequency of the signal. Bulk precipitation of dissolved mineral ions generally becomes more efficient with higher frequencies (i.e. frequencies greater than 3,000 Hz). However, self-induction in the solenoid system increases with frequency under Faraday s Law, negating any benefit gained from the increased frequency. In practice, the frequency in the solenoid-coil system is limited to 500 to 3,000 Hz. Since it is not efficient to use high frequencies in large-diameter pipe applications (i.e., greater than 6 inches in diameter), solenoid-coil systems are not desirable.
From the foregoing, it is apparent that existing PWT methods that utilize indirect electrical fields for the reduction of mineral scaling leave something to be desired. There remains a need to develop an effective means for reducing the formation of mineral scale in desalination systems using electric fields.